We love the keys! Key West is expensive but the rest not so bad. We like to stay near Marathon but up to Islamorada is nice too, lots of smaller ma & pa type accommodations. Key West is a blast for a few days but I would not want to spend a week there, be sure to spend at least one night though. If you like bottom fishing check out the Marathon Lady out of Marathon, there is a restaurant next to the dock that will cook your catch (love deep fried grunts).
Some places we have stayed and can comment on;
White Sands Inn, Mile Marker 58 ocean side near Marathon
Gulf View Waterfront Resort, MM59 gulf side near Marathon
Conch Key Cottages, MM62 ocean side
Blue waters Motel, about MM50 in Marathon gulf side
Lime Tree Bay, MM68.5 on Long Key gulf sidehttp://www.fla-keys.com/